PSA (polymer sustained alignment) liquid crystal display devices have a structure in which a polymer structure is disposed in a cell so as to control the pretilt angle of liquid crystal molecules and have been developed as liquid crystal display elements because of rapid response and high contrast.
The PSA liquid crystal display element is produced by injecting a polymerizable composition, which is composed of a liquid crystal compound and a polymerizable compound, between substrates and polymerizing the polymerizable compound while a voltage is applied and liquid crystal molecules are aligned so as to immobilize the alignment of the liquid crystal molecules. Known causes of image-sticking, which is a display defect of PSA liquid crystal display element, are impurities and a change in alignment of liquid crystal molecules (change in pretilt angle).
The image-sticking due to impurities occurs because of remaining polymerizable compounds resulting from incomplete polymerization and a polymerization initiator added for the purpose of facilitating the polymerization. Therefore, it is necessary that the amount of remaining polymerizable compound after the polymerization be minimized and the amount of addition of the polymerization initiator be reduced. For example, if a large amount of polymerization initiator is added such that the polymerization proceeds completely, the voltage holding rate of a display element is reduced because of remaining polymerization initiator, and the display quality is adversely affected. Also, if the amount of polymerization initiator used at the time of polymerization is reduced in order to suppress reduction in the voltage holding rate, the polymerizable compound remains because the polymerization does not completely proceed, and an occurrence of image-sticking due to the remaining polymerizable compound is unavoidable. Meanwhile, in order to completely cure the polymerizable compound in a small amount of addition of the polymerization initiator and to reduce the amount of remaining polymerizable compound, there is a method in which a large amount of energy is applied by intense ultraviolet irradiation for a long lime during polymerization. However, in this case, upsizing of a production apparatus and reduction in the production efficiency are caused and, in addition, degradation of a liquid crystal material due to ultraviolet rays and the like occur. Therefore, regarding the liquid crystal composition containing a polymerizable compound in the related art, it is difficult to reduce the amount of remaining of both uncured polymerizable compound and polymerization initiator at the same time.
Also, an occurrence of image-sticking resulting from a change in the pretilt angle of liquid crystal molecules is known. That is, the cause of image-sticking is a change in the pretilt angle when the display element continues to display the same pattern for a long time because of differences in cured materials of the polymerizable compound. In this case, a polymerizable compound that forms an appropriate polymer is required.
In order to prevent image-sticking, construction of a display element by using a polymerizable compound having a structure of 1,4-phenylene group or the like as a ring structure (refer to PTL 1) and construction of a display element by using a polymerizable compound having a biaryl structure (refer to PTL 2) have been disclosed. However, these polymerizable compounds have low compatibility with liquid crystal compounds and precipitation of the polymerizable compounds occurs when liquid crystal compositions are prepared. Therefore, it is difficult to apply these polymerizable compounds to practical liquid crystal compositions.
Also, in order to prevent image-sticking by improving the rigidity of a polymer, construction of a display element by using a mixed liquid crystal composition of a bifunctional polymerizable compound and trifunctional or higher polymerizable compound, e.g., dipentaerythritol pentaacrylate or dipentaerythritol hexaacrylate, (refer to PTL 3) has been proposed. However, dipentaerythritol pentaacrylate and dipentaerythritol hexaacrylate have no ring structure in the molecule and, therefore, there is a problem that sufficient alignment stability is not obtained because the affinity for liquid crystal compounds is weak and a force for regulating alignment is weak. Meanwhile, these polymerizable compounds require addition of polymerization initiators at the time of polymerization, and if the polymerization initiator is not added, the polymerizable compounds remain after the polymerization.
Consequently, it is difficult to sufficiently provide the characteristics, e.g., image-sticking characteristics of the display element, the alignment stability, the stability of the composition in which precipitation does not occur, and the production efficiency when PSA liquid crystal display element is produced, required of the liquid crystal composition containing a polymerizable compound. Therefore, further improvement is necessary.
As described above, attempts have been made to solve the display defect problem associated with the alignment of the liquid crystal molecules in the PSA display element having useful display performance (contrast and response speed) by using the polymerizable compound. Meanwhile, some components of the liquid crystal composition constituting the PSA display element are not suitable for use in the PSA display element. In particular, a liquid crystal composition containing a liquid crystal compound having an alkenyl group side chain useful for reducing viscosity so as to improve the response performance has been disclosed and is useful for reducing response speed of the VA display element (refer to PTL 4). However, there is a new problem related to alignment control, that is, after polymerization of the polymerizable compound, which is a production process of the PSA display element, provision of a pretilt angle to the liquid crystal molecules is hindered. In the case where an appropriate pretilt angle is not provided to the liquid crystal molecules, the movement direction of the liquid crystal molecules at the time of driving cannot be regulated and problems occur, for example, liquid crystal molecules do not fall in the predetermined direction so as to degrade the contrast, and the response speed is decreased.
As described above, it is necessary to ensure the performance, e.g., high contrast, rapid response, and high voltage holding rate, required of VA and other vertically aligned display elements and, in addition, items, e.g., generation of an appropriate pretilt angle and stability over time of the pretilt angle, required of PSA display element at the same time.